The parametric array uses the nonlinear response of a transmission medium such as air or water to convert or demodulate ultrasonic frequency waves into audio frequency waves of any audio frequency signals modulated onto the ultrasonic waves. This phenomenon is useful to direct a beam of ultra sound having audio modulated thereon to a specified region where it is demodulated in the medium and can be heard.
In a typical case of a parametric array in air, the audible signal will typically be from voice, music or other normal audio frequency source. Prior to or subsequent to the conversion or modulation into the ultrasound frequency ranges, some form of signal processing is typically undertaken. This is undertaken to compensate for the non-flat frequency response of typical ultrasound transducers, the transducer nonlinearity, environmental conditions of temperature and humidity and the position of the listening recipient among other effects that prevent a faithful reproduction of the original sound to the listener. The response of air to ultrasound is also nonlinear and may need compensation prior to the actual ultrasound emission.
The form of modulation typically employed provides a signal envelope on an ultrasonic carrier. The carrier and envelope have different response characteristics to the type of signal processing and the non-flat and nonlinear characteristics of the environment and devices used in this type of ultrasonic sound beaming from source to listener and a resulting lack of alignment.